Week 2 - Diagnosing Heart Conduction Using ECG

Question 1

What is an ECG?

Answer 1

ElectroCardioGram
a graph to show the measure of waves of electrical activity that pass across the heart

Question 2

What do each of the letters in an ECG represent?

Answer 2

P: atria depolarisation
Q: septum depolarisation (not always present)
RS: ventricle depolarisation
T: ventricle repolarisation
U: septum repolarisation (often not present)

Question 3

What do the peaks represent in an ECG?

Answer 3

• a change in voltage occurring
• can be both positive or negative
• a flat line thus does not mean the voltage is 0
• but that it has not changed

Question 4

What does the PR interval represent?

Answer 4

• before the P wave starts until Q or R begins (R because Q is not always present)
• atria depolarisation until before the ventricles depolarise

Question 5

Why is the SAN conduction not present in an ECG?

Answer 5

• the ECG is measured from a patient’s skin
• the SAN does not consist of many cells
• thus the signal sent is not strong enough by the time it reaches the skin for it to be detected

Question 6

What determines the up/down direction of the signals in an ECG?

Answer 6

• depolarisation is positive
• repolarisation is negative
• going downwards to the right is positive
• going upwards to the left is negative
• negative and negative make a positive and etc.

Question 7

What does the PR segment represent?

Answer 7

• after the P wave ends until before Q or R begins
• when the conduction is at the AVN

Question 8

What does the QT interval represent?

Answer 8

• when Q starts until after the T wave ends
• the ventricles depolarising and repolarising

Question 9

What does the ST segment represent?

Answer 9

• after the S wave ends until before the T wave begins
• ventricular contraction

Question 10

What are AV blocks?

Answer 10

when there is abnormal or absent conduction at the AVN

Question 11

What are the common AV blocks?

Answer 11

1^st degree: conduction is slowed, not interrupted
2^nd degree: conduction is partially interrupted, proceeds sometimes
3^rd degree: conduction is stopped

Question 12

How does a 1st degree AV block differ from normal?

Answer 12

• longer PR interval
• above 200ms

Question 13

What are the types of 2^nd degree blocks?
How do they differ from normal?

Answer 13

**mobitz I*: PR interval gets longer with each beat until a QRS complex goes missing
- process resets
mobitz II: steady P-interval but several QRS complexes missing
2:1 block: every other QRS complex is skipped

Question 14

How and why does a 3rd degree AV block differ from normal?

Answer 14

• no correspondence between P and QRS complex
  -QRS complexes are very infrequent
• less than 40 bpm
• conduction to the AVN is completely stopped so AVN creates its own beat with its own pacemaker cells
• no communication between this conduction and the one from the SAN so there is o correspondence between the waves in the ECG
• AVN pacemaking is much slower making the QRS complexes very infrequent

Question 15

What are other basic cardiac issues and how do they differ from normal?

Answer 15

arrhythmia: normal beats but SAN sends out signals at irregular intervals
- TS interval changes
tachycardia: fast heartbeat
- can be atrial/ ventricular/ both
- short frequent beats
flutter: tachycardia at very high rates
bradycardia: slow heartbeat
- can be atrial/ ventricular/ both
- longer infrequent beats
fibrillation: constant uncoordinated beating
- can be atrial/ ventricular/ both
- beats keep happening messily

Question 16

Why does fibrillation happen?

Answer 16

• local or global re-entry
• re-excites nearby muscles
• keeps contracting
• Bundle of Kent (Bypass Tract)
• passage through cardiomyocytes and cardiac skeleton
• passes AVN and contracts ventricles too early and not uniformly spread
• occurs during Wolf Parkinson white syndrome